Grammar guide Updated 2026-09-30
Modal Verbs of Deduction: Must, Might and Can't
Modal verbs of deduction are how a speaker shows certainty before the answer is known. The ladder runs from must at the top, down through should, may, might and could, to can't at the bottom. Class trouble comes from three places: putting the wrong strength on the ladder, forgetting the perfect form for the past, and reaching for mustn't when the meaning is can't.
The certainty ladder
Draw it as a vertical line rather than a list. Everything above the middle is the speaker leaning towards yes; everything below is leaning towards no. The middle is where most real conversation happens, which is why might is worth more class time than must.
| Strength | Modal | Example |
|---|---|---|
| almost certain | must | She must be at work — her car is outside. |
| expect it to be true | should | He should be home by now. |
| possible | may / might / could | They might be stuck in traffic. |
| almost certainly not | can't | That can't be right. |
Two habits are worth naming. Must in this meaning is not an obligation; it is a conclusion, and it almost always follows a reason, spoken or unspoken. Could on the ladder of possibility sounds softer than might in British teaching materials, but the difference is small and not worth a lesson of its own.
Deduction about the present: modal plus bare verb
The pattern is modal + infinitive without to: He must be tired, not He must to be tired. To describe something happening right now, add be + -ing: The light is on — he must be working late. Negatives are built with can't: She can't be serious.
- Certain: You've been driving all day. You must be exhausted.
- Expected: The shop should be open by nine.
- Possible: He might be in a meeting; he isn't answering.
- Impossible: It can't be Tuesday — we had Tuesday yesterday.
Deduction about the past: modal plus have plus participle
Once the moment has gone, the bare verb cannot carry the meaning on its own. English adds the perfect: modal + have + past participle. The strength of the modal does not change; only the time frame does.
| Form | Example | Meaning |
|---|---|---|
| must have + participle | Someone must have thrown a stone. | The only sensible explanation. |
| might have + participle | They might have taken the wrong road. | One possibility among several. |
| can't have + participle | She can't have finished already. | Impossible, given the evidence. |
The participle is a real trap, because the modal steals the tense and learners drop the third form: must have wrote has to become must have written. Ten quick items on irregular participles fixes more of this than any explanation. The past continuous guide is a useful page to pair with this one, since the two structures describe the same scenes from different angles.
Why the negative is can't and not mustn't
This is the single most useful correction in the lesson. Mustn't is about rules: You mustn't smoke here. Can't is about logic: He can't have taken the car — the keys are on the table. A learner who says He mustn't be at home has accidentally turned a conclusion into a prohibition, and the sentence sounds like a rule about where he is allowed to stand.
Keep the pair on the board and ask for one example of each before moving on. The distinction sticks faster when the two sentences are about the same person: She mustn't park there (a rule) and She can't be parking there (a conclusion).
maybe, may be and might be
Two words and one phrase cause a lot of written slips. Maybe is an adverb and takes the front of the sentence: Maybe she's at home. May be is a modal plus the verb be and sits inside the sentence: She may be at home. Writing May be she is at home is not standard English. Might and may are close enough to be taught together; after both, the next verb stays in the bare form: She might be at home, never She might is at home. The relative clauses guide follows the same habit of putting two easily confused forms side by side on one board.
Where the errors come from
- He mustn't be at work. A deduction takes can't: He can't be at work — it's Sunday.
- He must to be tired. No to after a modal: He must be tired.
- She must have wrote it. Use the participle: She must have written it.
- May be he is late. One word at the front of a sentence: Maybe he is late.
- He must be at home, no? Short replies keep the modal: He must be, mustn't he? or simply I'm sure he is.
Three drills that build the strength sense
- Clue and conclusion. Read one clue: the office lights are off, there is an empty cup on the desk, the car has a flat tyre. The class answers with must, might or can't and gives the reason out loud. Twenty clues fills a lesson.
- Same photo, three sentences. Show one picture — a crowded platform, an empty restaurant — and ask for one must, one might and one can't sentence about it. The ladder becomes visible in the answers.
- Yesterday's mystery. Pairs invent a small event with three pieces of evidence, then write three past deductions with must have, might have and can't have before reading them to another pair to guess the event.
Practice that keeps the pattern alive
Short daily practice beats one long session, and the modal forms survive only if learners hear them often. The classroom grammar drills page has timed rounds that fit a forty-minute lesson, while the future tenses guide covers the will and going to forms that carry deduction into the future with an adverb such as probably or definitely. The quantifiers guide is the natural next step, because certainty and quantity both ask learners to judge how much of a claim they can support.
Browse the classroom drills Past continuous guide Teach quantifiers next
Frequently asked questions
What is the difference between must, might and can't?
They show how sure the speaker is. Must means almost certain: She must be at work. Might, may and could mean possible: They might be stuck in traffic. Can't means almost certainly not: That can't be right.
Why do we use can't and not mustn't for deduction?
Mustn't gives a rule or a prohibition: You mustn't smoke here. Can't shows that something is logically impossible: He can't have taken the car, the keys are still on the table.
How do I make a deduction about the past?
Use the modal plus have plus the past participle: someone must have thrown a stone, she can't have finished already, they might have taken the wrong road. The negative past form is can't have, not mustn't have.
What is the difference between maybe and may be?
Maybe is an adverb and starts a sentence: Maybe she is at home. May be is a modal plus the verb be inside a sentence: She may be at home. After might or may the next verb is always the bare infinitive.
Can must have be used for the future?
No. Deduction with must, might and can't is about the present or the past. For the future, English uses will or going to with an adverb such as probably or definitely.
Keep reading
Every guide on this blog is paired with a free drill: the same rules, applied question by question, with explanations and a printable worksheet.